There is known a vehicular automatic transmission of a type including two rotary shafts which are coaxial with each other and which are splined to each other at their adjacent axial end portions to be rotated together. This automatic transmission has a lubrication system having lubrication passages formed through the two rotary shafts so as to extend in their axial direction. The lubrication passages in the two rotary shafts are held in communication with each other at the spline-connected adjacent axial end portions. A lubricating oil is introduced into the lubrication passages through an oil inlet hole formed at one of the opposite axial ends of one of the two rotary shafts rotated together. The lubrication system further has a plurality of oil outlet holes from which the lubricating oil is fed out from the lubrication passages to various lubricated parts of the automatic transmission. JP-7-47985 B2 discloses such a lubrication system for a vehicular automatic transmission, which permits improved lubricating efficiency.
In the automatic transmission of the type described above, there exists a gap between the spline teeth formed in the splined-connected adjacent axial end portions of the two rotary shafts, so that the lubricating oil flows from the lubrication passages through this gap. This flow of the lubricating oil through the gap causes a loss of the lubricating oil for the lubricated parts corresponding to one of the two rotary shafts which is not provided with the inlet indicated above. Accordingly, the amount of the lubricating oil to be fed to those lubricated parts tends to be insufficient. This problem is solved by providing the spline-connected adjacent axial end portions of the two rotary shafts with a suitable sealing device arranged to prevent the oil flow through the gap between the spline teeth. However, the provision of the sealing device requires a support structure for holding the sealing member, which undesirably increases the size and number of components of the vehicular automatic transmission, and accordingly increases the number of manufacturing process steps and the cost of manufacture of the automatic transmission.